Question: Expand. If necessary, combine like terms. $(x+3)(x-3)=$
Solution: Notice that this expression has the following special form: $(a+b)(a-b)$ This form expands to what we call "a difference of squares": $( a+ b)( a- b)= a^2- b^2$ Using the above pattern, we get: $\begin{aligned} ( x+ 3)( x- 3)&= x^2- 3^2 \\\\ &=x^2-9 \end{aligned}$